The present invention generally relates to an operational amplifier, and more particularly to a CMOS differential operational amplifier having characteristics of a low operating voltage, a high voltage gain and a high speed.
Recently, due to the rapid development of VLSI technology, a variety of circuits can be integrated into one chip in which digital circuits and analog circuits are mixed. Operational amplifiers are being used widely in communication circuits, switched capacitor filters, A/D and D/A convertors, etc. Particularly, in VLSI circuits, the operational amplifiers are essential elements. Also, the development of semiconductor IC technology brings about a reduction of device size, and thus results in improvement of integration. However, reduction of the device size gives rise to many problems in fabricating semiconductor devices. In particular, it is a serious problem that an internal voltage of the device decreases due to increment of the impurity doping concentration. Therefore, in circuits using such devices having a decreased internal voltage, the power source voltage must be decreased. The decrement of the power source voltage induces decreasing of the voltage gain and the output swing of the operational amplifier.